But a new scientific study confirms what biologists have long known: the biggest danger to the condor currently is found on the ground, not in the skies, in the form of lead bullet-ridden carcasses of the carrion the birds eat.

As the wild population of California condors dwindled to 22 birds in 1987, biologists captured the survivors and put them in a captive breeding program, primarily at the Los Angeles and San Diego zoos. Between October 1992, when the bird was reintroduced into the wild, and Dec. 31, 2009, 135 free-range condors have died. (The current population stands at 400 with half of the condors living in the wild in California, Arizona and Baja, Mexico).

Autopsies in 76 cases where a cause of death could be determined showed that lead poisoning accounted for 67% of the adult condor deaths and 26% of juvenile fatalities, according to the study published in the Journal of Wildlife Diseases this month and written by scientists from the San Diego Zoo, U.S. Fish and Wildlife Service and several universities.

“The mortality factors thought to be important in the decline of the historical California condor population, particularly lead poisoning, remain the most important documented mortality factors today,” the study’s authors wrote. “Without effective mitigation, these factors can be expected to have the same effects on the sustainability of the wild populations as they have had in the past.”

A ban on the use of lead bullets in the condor’s range in California took effect in July 2008 but biologists told me condors continue to be diagnosed with lead poisoning from shell casings.

The condor death rate would be far higher if the biologists didn’t constantly intervene to detect and treat lead poisoning. Twice a year, Fish and Wildlife Service biologists capture the entire wild population at condor refuges in California to test the birds for lead exposure.

When I recently visited the Bitter Creek National Wildlife Refuge near the Tehachapi Mountains, Jesse Grantham, a veteran condor biologist with the Fish and Wildlife Service, told me that 54 birds had undergone some 90 treatments for lead poisoning between January 2010 and October 2011. Four birds have died from lead poisoning since January 2010 with lead suspected but not yet confirmed in the death of three others.

“It’s still a problem,” said Grantham as two condors soared through a canyon, surfing thermal currents. “Some of those birds have been in the L.A. Zoo for treatment two or three times. So we’re still heavily managing the birds”

Other condors have been killed in collisions with power lines while young birds have died after ingesting so-called micro trash – small pieces of plastic and other manmade garbage picked up by adult condors and brought back to the nest.

But in a sign of a return to the natural cycle of life and death, in the fall of 2010 three juvenile condors succumbed to an attack by a mountain lion just across the canyon where Grantham and I watched the two condors soaring.

I'm the environment editor at Forbes. Before joining Forbes in April 2011, I wrote about all things green and tech as a contributor to The New York Times, a senior editor at Fortune and an assistant managing editor at Business 2.0 magazine. I previously was the business edit...